Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.
Measurements of optical components and in particular the human eye have been addressed by a range of different instruments which have been able to provide information regarding different aspects of the eye's morphology and function as well as identification of various anomalies. The Shack-Hartmann technique is perhaps the most widely used method for measuring ocular wavefront aberrations. As shown in schematic form in FIG. 1, an incoming known beam 2 is transmitted through a beam splitter 4 to an eye under test 6, where the beam is focused by the eye's optical power 8 onto or close to the retina 10. A small reflected component is then collimated by the eye's optical power and separated from the incoming beam 2 by the beam splitter 4 to form an outgoing wavefront 12 which contains information on residual optical power and aberrations of the eye 6. The outgoing wavefront 12 is analysed with a Shack-Hartmann analyser 14 which, as shown in the enlargement in FIG. 1a, consists of a micro lens array 16 that samples the wavefront across a predetermined grid and focuses it onto a focal plane array 18. The positions of the image spots 20 from each micro lens can be used to estimate the slope 22 of wavefront 12 at each sampling point. If the slopes can be determined with sufficient accuracy and if the changes between the sampling points are not too great then it is possible to reconstruct the actual phase 24 of the wavefront at each of the sampling points. An advantage of the Shack-Hartmann technique is that it is self-referencing, improving robustness. A drawback is that as the wavefront slope becomes larger the image spots 20 begin to overlap, which limits the maximum level of aberration that can be measured. Furthermore the sampling resolution is limited by the micro lens array 16, which may for example comprise 30×30 micro lenses on a 250 μm pitch.
Interest in peripheral vision has been increasing in recent years, partly because of the suggestion that it may influence eye growth and myopia development. A scanning implementation of the Shack-Hartmann technique has been reported by Jaeken et al (Optics Express 19(8), 7903-7913, 11 Apr. 2011), capable of measuring over 80 degrees of visual field with 1 degree resolution in 1.8 s. Although relatively fast, this scanning method may still be compromised by changes in the subject eye during the measurement time, and it also suffers from the general limitations of the Shack-Hartmann technique described above.
In the shearing interferometry technique (Dubra et al Applied Optics 44(7), 1191-1199, 1 Mar. 2005) the gradient of a wavefront is inferred from the interference of laterally shifted copies of the wavefront. Like the Shack-Hartmann technique, shearing interferometry is a robust self-referential approach suitable for ocular examination and has the advantage of higher spatial resolution. Whilst simple in principle, the implementation is complicated by the requirement for the wavefront gradient to be measured simultaneously in more than one direction, see for example Kühn et al (Optics Express 15(2), 7231-7242, 11 Jun. 2007) which describes the use of a holographic grating structure to produce shifted copies of a wavefront in orthogonal lateral directions. Although these approaches are robust to patient movement they are reliant on the interference of two copies of the sample signal. This causes little difficulty if the sample signals are relatively intense, as in the specular corneal tear film reflections analysed in Dubra et al for example, but is of much greater concern when measuring the considerably weaker retinal reflections.